Encoder, decoder and methods for encoding and decoding data segments representing a time-domain data stream

ABSTRACT

An apparatus for decoding data segments representing a time-domain data stream, a data segment being encoded in the time domain or in the frequency domain, a data segment being encoded in the frequency domain having successive blocks of data representing successive and overlapping blocks of time-domain data samples. The apparatus includes a time-domain decoder for decoding a data segment being encoded in the time domain and a processor for processing the data segment being encoded in the frequency domain and output data of the time-domain decoder to obtain overlapping time-domain data blocks. The apparatus further includes an overlap/add-combiner for combining the overlapping time-domain data blocks to obtain a decoded data segment of the time-domain data stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/637,256, filed 3 Mar., 2015, which is a divisionalapplication of U.S. patent application Ser. No. 14/250,306, filed 10Apr. 2014, which issued as U.S. Pat. No. 9,043,202 on 26 May 2015, whichis a continuation application of U.S. patent application Ser. No.13/924,441 filed 21 Jun. 2013, which issued as U.S. Pat. No. 8,812,305on 19 Aug. 2014, which is a divisional application of U.S. patentapplication Ser. No. 12/518,627 filed 17 Dec. 2009, which issued as U.S.Pat. No. 8,818,796 on 26 Aug. 2014, which is a U.S. national entry ofPCT Patent Application Serial No. PCT/EP2007/010665 filed 7 Dec. 2007,and claims priority to U.S. Provisional Patent Application No.60/869,670 filed on 12 Dec. 2006, each of which is incorporated hereinin its entirety by this reference thereto.

BACKGROUND OF THE INVENTION

The present invention is in the field of coding, where differentcharacteristics of data to be encoded are utilized for coding rates, asfor example in video and audio coding.

State of the art coding strategies can make use of characteristics of adata stream to be encoded. For example, in audio coding, perceptionmodels are used in order to compress source data almost withoutdecreasing the noticeable quality and degradation when replayed. Modernperceptual audio coding schemes, such as for example, MPEG-2/4 AAC(MPEG=Moving Pictures Expert Group, AAC=Advanced Audio Coding), cf.Generic Coding of Moving Pictures and Associated Audio: Advanced AudioCoding, International Standard 13818-7, ISO/IEC JTC1/SC29/WG11 MovingPictures Expert Group, 1997, may use filter banks, such as for examplethe Modified Discrete Cosine Transform (MDCT), for representing theaudio signal in the frequency domain.

In the frequency domain quantization of frequency coefficients can becarried out, according to a perceptual model. Such coders can provideexcellent perceptual audio quality for general types of audio signalsas, for example, music. On the other hand, modern speech coders, suchas, for example, ACELP (ACELP=Algebraic Code Excited Linear Prediction),use a predictive approach, and in this way may represent theaudio/speech signal in the time domain. Such speech coders can model thecharacteristics of the human speech production process, i.e. the humanvocal tract and, consequently, achieve excellent performance for speechsignals at low bit rates. Conversely, perceptional audio coders do notachieve the level of performance offered by speech coders for speechsignals coded at low bit rates, and using speech coders to representgeneral audio signals/music results in significant quality impairments.

Conventional concepts provide a layered combination in which all partialcoders are active, i.e. time-domain and frequency-domain encoders, andthe final output signal is calculated by combining the contributions ofthe partial coders for a given processed time frame. A popular exampleof layered coding are MPEG-4 scalable speech/audio coding with a speechcoder as the base layer and a filterbank-based enhancement layer, cf.Bernhard Grill, Karlheinz Brandenburg, “A Two- or Three-Stage Bit-RateScalable Audio Coding System”, Preprint Number 4132, 99^(th) Conventionof the AES (September 1995).

Conventional frequency-domain encoders can make use of MDCT filterbanks.The MDCT has become a dominant filterbank for conventional perceptualaudio coders because of its advantageous properties. For example, it canprovide a smooth cross-fade between processing blocks. Even if a signalin each processing block is altered differently, for example due toquantization of spectral coefficients, no blocking artifacts due toabrupt transitions from block to block occur because of the windowedoverlap/add operations. The MDCT uses the concept of time-domainaliasing cancellation (TDAC).

The MDCT is a Fourier-related transform based on the type-IV discretecosine transform, with the additional property of being lapped. It isdesigned to be performed in consecutive blocks of a larger data set,where subsequent blocks are overlapped so that the last half of oneblock coincides with the first half of the next block. This overlapping,in addition to an energy-compaction quality of the DCT, makes the MDCTespecially attractive for signal compression applications, since ithelps to avoid said artifacts stemming from the block boundaries. As alapped transform, the MDCT is a bit unusual compared to otherFourier-related transforms in that it has half as many outputs asinputs, instead of the same number. In particular, 2N real numbers aretransformed into N real numbers, where N is a positive integer.

The inverse MDCT is also known as IMDCT. Because there are differentnumbers of inputs and outputs, at first glance it might seem that theMDCT should not be invertible. However, perfect invertibility isachieved by adding the overlap IDMCTs of subsequent overlapping blocks,causing the errors to cancel and the original data to be retrieved, i.e.achieving TDAC.

Therewith, the number of spectral values at the output of a filterbankis equal to the number of time-domain input values at its input which isalso referred to as critical sampling.

An MDCT filterbank provides a high-frequency selectivity and enables ahigh coding gain. The properties of overlapping of blocks and criticalsampling can be achieved by utilizing the technique of time-domainaliasing cancellation, cf. J. Princen, A. Bradley, “Analysis/SynthesisFilter Bank Design Based on Time Domain Aliasing Cancellation”, IEEETrans. ASSP, ASSP-34(5):1153-1161, 1986. FIG. 4 illustrates theseeffects of an MDCT. FIG. 4 shows an MDCT input signal, in terms of animpulse along a time axis 400 at the top. The input signal 400 is thentransformed by two consecutive windowing and MDCT blocks, where thewindows 410 are illustrated underneath the input signal 400 in FIG. 4.The back transformed individual windowed signals are displayed in FIG. 4by the time lines 420 and 425.

After the inverse MDCT, the first block produces an aliasing componentwith positive sign 420, the second block produces an aliasing componentwith the same magnitude and a negative sign 425. The aliasing componentscancel each other after addition of the two output signals 420 and 425as shown in the final output 430 at the bottom of FIG. 4.

In “Extended Adaptive Multi-Rate—Wideband (AMR-WB+) codec”, 3GPP TS26.290V6.3.0, 2005-06, Technical Specification theAMR-WB+(AMR-WB=Adaptive Multi-Rate Wideband) codec is specified.According to section 5.2, the encoding algorithm at the core of theAMR-WB+ codec is based on a hybrid ACELP/TCX (TCX=Transform codedExcitation) model. For every block of an input signal the encoderdecides, either in an open loop or a closed loop mode which encodingmodel, i.e. ACELP or TCX, is best. The ACELP model is a time-domain,predictive encoder, best suited for speech and transient signals. TheAMR-WB encoder is used in ACELP modes. Alternatively, the TCX model is atransform based encoder, and is more appropriate for typical musicsamples.

Specifically, the AMR-WB+ uses a discrete Fourier transform (DFT) forthe transform coding mode TCX. In order to allow a smooth transitionbetween adjacent blocks, a windowing and overlap is used. This windowingand overlap is useful both for transitions between different codingmodes (TCX/ACELP) and for consecutive TCX frames. Thus, the DFT togetherwith the windowing and overlap represents a filterbank that is notcritically sampled. The filterbank produces more frequency values thanthe number of new input samples, cf. FIG. 4 in 3GPP TS 26.290V6.3.0(3GPP=Third Generation Partnership Project, TS=Technical Specification).Each TCX frame utilizes an overlap of ⅛ of the frame length which equalsthe number of new input samples. Consequently, the corresponding lengthof the DFT is 9/8 of the frame length.

Considering the non-critically sampled DFT filterbank in the TCX, i.e.the number of spectral values at the output of the filterbank is largerthan the number of time-domain input values at its input, this frequencydomain coding mode is different from audio codecs such as AAC(AAC=Advanced Audio Coding) which utilizes an MDCT, a critically sampledlapped transform.

The Dolby E codec is described in Fielder, Louis D.; Todd, Craig C.,“The Design of a Video Friendly Audio Coding System for DistributingApplications”, Paper Number 17-008, The AES 17^(th) InternationalConference: High-Quality Audio Coding (August 1999) and Fielder, LouisD.; Davidson, Grant A., “Audio Coding Tools for Digital TelevisionDistribution”, Preprint Number 5104, 108^(th) Convention of the AES(January 2000). The Dolby E codec utilizes the MDCT filterbank. In thedesign of this coding, special focus was put on the possibility toperform editing in the coding domain. To achieve this, specialalias-free windows are used. At the boundaries of these windows asmooth-cross fade or splicing of different signal portions is possible.In the above-referenced documents it is, for example, outlined, cf.section 3 of “The Design of a Video Friendly Audio Coding System forDistribution Applications”, that this would not be possible by simplyusing the usual MDCT windows which introduce time-domain aliasing.However, it is also described that the removal of aliasing comes at thecost of an increased number of transform coefficients, indicating thatthe resulting filterbank does not have the property of critical samplinganymore.

SUMMARY

According to an embodiment, an apparatus for decoding data segmentsrepresenting a time-domain data stream, a data segment being encoded inthe time domain or in the frequency domain, a data segment being encodedin the frequency domain having successive blocks of data representingsuccessive and overlapping blocks of time-domain data samples, whereintwo consecutive time-domain data blocks of which one being encoded inthe frequency domain and one being encoded in the time domain having atime domain overlapping region, may have: a time-domain decoder fordecoding a data segment being encoded in the time domain to acquireoutput data of the time-domain decoder; a processor for processing thedata segments being encoded in the frequency domain and the output dataof the time-domain decoder to acquire overlapping time-domain datablocks for the time domain overlapping region; and anoverlap/add-combiner for combining the overlapping time-domain datablocks in the time domain overlapping region to acquire a decoded datasegment of the time-domain data stream for the time domain overlappingregion.

According to another embodiment, a method for decoding data segmentsrepresenting a time-domain data stream, a data segment being encoded inthe time domain or in the frequency domain, a data segment being encodedin the frequency domain having successive blocks of data representingsuccessive and overlapping blocks of time-domain data samples, whereintwo consecutive time-domain data blocks of which one being encoded inthe frequency domain and one being encoded in the time domain having atime domain overlapping region, may have the steps of: decoding a datasegment being encoded in the time domain to acquire output data of thetime-domain decoder; processing the data segment being encoded in thefrequency domain and the output data of the time-domain decoder toacquire overlapping time-domain data blocks for the time domainoverlapping region; and combining the overlapping time-domain datablocks in the time domain overlapping region to acquire a decoded datasegment of the time-domain data stream for the time domain overlappingregion.

According to another embodiment, a computer program may have: a programcode for performing the method for decoding data segments representing atime-domain data stream, a data segment being encoded in the time domainor in the frequency domain, a data segment being encoded in thefrequency domain having successive blocks of data representingsuccessive and overlapping blocks of time-domain data samples, whereintwo consecutive time-domain data blocks of which one being encoded inthe frequency domain and one being encoded in the time domain having atime domain overlapping region, wherein the method may have the stepsof: decoding a data segment being encoded in the time domain to acquireoutput data of the time-domain decoder; processing the data segmentbeing encoded in the frequency domain and the output data of thetime-domain decoder to acquire overlapping time-domain data blocks forthe time domain overlapping region; and combining the overlappingtime-domain data blocks in the time domain overlapping region to acquirea decoded data segment of the time-domain data stream for the timedomain overlapping region, when the program code runs on a computer.

According to another embodiment, an apparatus for generating an encodeddata stream based on a time-domain data stream, the time-domain datastream having samples of a signal, may have: a segment processor forproviding data segments from the data stream, two consecutive datasegments having a first or a second overlapping region, the secondoverlapping region being smaller than the first overlapping region; atime-domain encoder for encoding a windowed data segment in the timedomain; a frequency-domain encoder for applying weights to samples ofthe time-domain data stream according to a first or second windowingfunction to acquire a windowed data segment, the first and secondwindowing functions being adapted to the first and second overlappingregions, the frequency-domain encoder being adapted for encoding awindowed data segment in the frequency domain; a time-domain dataanalyzer for determining a transition indication associated with a datasegment; and a controller for controlling the apparatus such that fordata segments having a first transition indication output data of thetime-domain encoder is included in the encoded data stream and for datasegments having a second transition indication, output data of thefrequency-domain encoder is included in the encoded data stream.

According to another embodiment, a method for generating an encoded datastream based on a time-domain data stream, the time-domain data streamhaving samples of a signal, may have the steps of: providing datasegments from the data stream, two consecutive data segments having afirst or a second overlapping region, the second overlapping regionbeing smaller than the first overlapping region; determining atransition indication associated with the data segments; encoding a datasegment in the time domain, and/or applying weights to samples of thetime-domain data stream according to a first or a second windowingfunction to acquire a windowed data segment, the first and secondwindowing functions being adapted to the first and second overlappingregions and encoding the windowed data segment in the frequency domainand; controlling such that for data segments having a first transitionindication output data being encoded in the time-domain is included inthe encoded data stream and for data segments having a second transitionindication output data being encoded in the frequency domain is includedin the encoded data stream.

According to another embodiment, a computer program may have: a programcode for performing the method for generating an encoded data streambased on a time-domain data stream, the time-domain data stream havingsamples of a signal, wherein the method may have the steps of: providingdata segments from the data stream, two consecutive data segments havinga first or a second overlapping region, the second overlapping regionbeing smaller than the first overlapping region; determining atransition indication associated with the data segments; encoding a datasegment in the time domain, and/or applying weights to samples of thetime-domain data stream according to a first or a second windowingfunction to acquire a windowed data segment, the first and secondwindowing functions being adapted to the first and second overlappingregions and encoding the windowed data segment in the frequency domainand; controlling such that for data segments having a first transitionindication output data being encoded in the time-domain is included inthe encoded data stream and for data segments having a second transitionindication output data being encoded in the frequency domain is includedin the encoded data stream, when the program code runs on a computer.

The present invention is based on the finding that a more efficientencoding and decoding concept can be utilized by using combinedtime-domain and frequency-domain encoders, respectively decoders. Theproblem of time aliasing can be efficiently combat by transformingtime-domain data to the frequency-domain in the decoder and by combiningthe resulting transformed frequency-domain data with the decodedfrequency-domain data received. Overheads can be reduced by adaptingoverlapping regions of overlap windows being applied to data segments tocoding domain changes. Using windows with smaller overlapping regionscan be beneficial when using time-domain encoding, respectively whenswitching from or to time-domain encoding.

Embodiments can provide a universal audio encoding and decoding conceptthat achieves improved performance for both types of input signals, suchas speech signals and music signals. Embodiments can take advantage bycombining multiple coding approaches, e.g. time-domain andfrequency-domain coding concepts. Embodiments can efficiently combinefilterbank based and time-domain based coding concepts into a singlescheme. Embodiments may result in a combined codec which can, forexample, be able to switch between an audio codec for music-like audiocontent and a speech codec for speech-like content. Embodiments mayutilize this switching frequently, especially for mixed content.

Embodiments of the present invention may provide the advantage that noswitching artifacts occur. In embodiments the amount of additionaltransmit data, or additionally coded samples, for a switching processcan be minimized in order to avoid a reduced efficiency during thisphase of operation. Therewith the concept of switched combination ofpartial coders is different from that of the layered combination inwhich all partial coders are active.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1a shows an embodiment of an apparatus for decoding;

FIG. 1b shows another embodiment of an apparatus for decoding;

FIG. 1c shows another embodiment of an apparatus for decoding;

FIG. 1d shows another embodiment of an apparatus for decoding;

FIG. 1e shows another embodiment of an apparatus for decoding;

FIG. 1f shows another embodiment of an apparatus for decoding;

FIG. 2a shows an embodiment of an apparatus for encoding;

FIG. 2b shows another embodiment of an apparatus for encoding;

FIG. 2c shows another embodiment of an apparatus for encoding;

FIG. 3a illustrates overlapping regions when switching betweenfrequency-domain and time-domain coding for the duration of one window;

FIG. 3b illustrates the overlapping regions when switching betweenfrequency-domain coding and time-domain coding for a duration of twowindows;

FIG. 3c illustrates multiple windows with different overlapping regions;

FIG. 3d illustrates the utilization of windows with differentoverlapping regions in an embodiment; and

FIG. 4 illustrates time-domain aliasing cancellation when using MDCT.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows an apparatus 100 for decoding data segments representing atime-domain data stream, a data segment being encoded in a time domainor in a frequency domain, a data segment being encoded in the frequencydomain having successive blocks of data representing successive andoverlapping blocks of time-domain data samples. This data stream could,for example, correspond to an audio stream, wherein some of the datablocks are encoded in the time domain and other ones are encoded in thefrequency domain. Data blocks or segments which have been encoded in thefrequency domain, may represent time-domain data samples of overlappingdata blocks.

The apparatus 100 comprises a time-domain decoder 110 for decoding adata segment being encoded in the time domain. Furthermore, theapparatus 100 comprises a processor 120 for processing the data segmentbeing encoded in the frequency domain and output data of the time-domaindecoder 110 to obtain overlapping time-domain data blocks. Moreover, theapparatus 100 comprises an overlap/add-combiner 130 for combining theoverlapping time-domain data blocks to obtain the decoded data segmentsof the time-domain data stream.

FIG. 1b shows another embodiment of the apparatus 100. In embodimentsthe processor 120 may comprise a frequency-domain decoder 122 fordecoding data segments being encoded in the frequency domain to obtainfrequency-domain data segments. Moreover, in embodiments the processor120 may comprise a time-domain to frequency-domain converter 124 forconverting the output data of the time-domain decoder 110 to obtainconverted frequency-domain data segments.

Furthermore, in embodiments the processor 120 may comprise afrequency-domain combiner 126 for combining the frequency-domainsegments and the converted frequency-domain data segments to obtain afrequency-domain data stream. The processor 120 may further comprise afrequency-domain to time-domain converter 128 for converting thefrequency-domain data stream to overlapping time-domain data blockswhich can then be combined by the overlap/add-combiner 130.

Embodiments may utilize an MDCT filterbank, as for example, used inMPEG-4 AAC, without any modifications, especially without giving up theproperty of critical sampling. Embodiments may provide optimum codingefficiency. Embodiments may achieve a smooth transition to a time-domaincodec compatible with the established MDCT windows while introducing noadditional switching artifacts and only a minimal overhead.

Embodiments may keep the time-domain aliasing in the filterbank andintentionally introduce a corresponding time-domain aliasing into thesignal portions coded by the time-domain codec. Thus, resultingcomponents of the time-domain aliasing can cancel each other out in thesame way as they do for two consecutive frames of the MDCT spectra.

FIG. 1c illustrates another embodiment of an apparatus 100. According toFIG. 1c the frequency-domain decoder 122 can comprise a re-quantizationstage 122 a. Moreover, the time-domain to frequency-domain converter 124can comprise a cosine modulated filterbank, an extended lappedtransform, a low delay filterbank or a polyphase filterbank. Theembodiment shown in FIG. 1c illustrates that the time-domain tofrequency-domain converter 124 can comprise an MDCT 124 a.

Furthermore, FIG. 1c depicts that the frequency-domain combiner 126 maycomprise an adder 126 a. As shown in FIG. 1 c, the frequency-domain totime-domain converter 128 can comprise a cosine modulated filterbank,respectively an inverse MDCT 128 a. The data stream comprisingtime-domain encoded and frequency-domain encoded data segment may begenerated by an encoder which will be further detailed below. Theswitching between frequency-domain encoding and time-domain encoding canbe achieved by encoding some portions of the input signal with afrequency-domain encoder and some input signal portions with atime-domain encoder. The embodiment of the apparatus 100 depicted inFIG. 1c illustrates the principle structure of a corresponding apparatus100 for decoding. In other embodiments the re-quantization 122 a and theinverse modified discrete cosine transform 128 a can represent afrequency-domain decoder.

As indicated in FIG. 1c for signal portions where the time-domaindecoder 110 takes over, the time-domain output of the time-domaindecoder 110 can be transformed by the forward MDCT 124 a. Thetime-domain decoder may utilize a prediction filter to decode thetime-domain encoded data. Some overlap in the input of the MDCT 124 aand thus some overhead may be introduced here. In the followingembodiments will be described which reduce or minimize this overhead.

In principle, the embodiment shown in FIG. 1c also comprises anoperation mode where both codecs can operate in parallel. In embodimentsthe processor 120 can be adapted for processing a data segment beingencoded in parallel in the time domain and in the frequency domain. Inthis way the signal can partially be coded in the frequency domain andpartially in the time domain, similar to a layered coding approach. Theresulting signals are then added up in the frequency domain, compare thefrequency-domain combiner 126 a. Nevertheless, embodiments may carry outa mode of operation which is to switch exclusively between the twocodecs and only have an advantageously minimum number of samples whereboth codecs are active in order to obtain best possible efficiency.

In FIG. 1c , the output of the time-domain decoder 110 is transformed bythe MDCT 124 a, followed by the IMDCT 128 a. In another embodiment,these two steps may be advantageously combined into a single step inorder to reduce complexity. FIG. 1d illustrates an embodiment of anapparatus 100 illustrating this approach. The apparatus 100 shown inFIG. 1d illustrates that the processor 120 may comprise a calculator 129for calculating overlapping time-domain data blocks based on the outputdata of the time-domain decoder 110. The processor 120 or the calculator129 can be adapted for reproducing a property respectively anoverlapping property of the frequency-domain to time-domain converter128 based on the output data of the time-domain decoder 110, i.e. theprocessor 120 or calculator 129 may reproduce an overlappingcharacteristic of time-domain data blocks similar to an overlappingcharacteristic produced by the frequency-domain to time-domain converter128. Moreover, the processor 120 or calculator 129 can be adapted forreproducing time-domain aliasing similar to time-domain aliasingintroduced by the frequency-domain to time-domain converter 128 based onthe output data of the time-domain decoder 110.

The frequency-domain to time-domain converter 128 can then be adaptedfor converting the frequency-domain data segments provided by thefrequency-domain decoder 122 to overlapping time-domain data blocks. Theoverlap/add-combiner 130 can be adapted for combining data blocksprovided by the frequency-domain to time-domain converter 128 and thecalculator 129 to obtain the decoded data segments of the time-domaindata stream.

The calculator 129 may comprise a time-domain aliasing stage 129 a as itis illustrated in the embodiment shown in FIG. 1e . The time-domainaliasing stage 129 a can be adapted for time-aliasing output data of thetime-domain decoder to obtain the overlapping time-domain data blocks.

For the time-domain encoded data a combination of the MDCT and the IMDCTcan make the process in embodiments much simpler in both structure andcomputational complexity as only the process of time-domain aliasing(TDA) remains in embodiments. This efficient process can be based on anumber of observations. The windowed MDCT of the input segments of 2Nsamples can be decomposed into three steps.

First, the input signal is multiplied by an analysis window.

Second, the result is then folded down from 2N samples to N samples. Forthe MDCT, this process implies that the first quarter of the samples iscombined, i.e. subtracted, in time-reversed order with the secondquarter of the samples, and that the fourth quarter of the samples iscombined, i.e. added, with the third quarter of the samples intime-reversed order. The result is the time-aliased, down-sampled signalin the modified second and third quarter of the signal, comprising Nsamples.

Third, the down-sampled signal is then transformed using an orthogonalDCT-like transform mapping N input to N output samples to form the finalMDCT output.

The windowed IMDCT reconstruction of an input sequence of N spectralsamples can likewise be decomposed into three steps.

First, the input sequence of N spectral samples is transformed using anorthogonal inverse DCT-like transform mapping N input to N outputsamples.

Second, the results unfolded from N to 2N samples by writing the inverseDCT transformed values into the second and third quarter of a 2N samplesoutput buffer, filling the first quarter with the time-reversed andinverted version of the second quarter, and the fourth quarter with atime-reverse version of the third quarter, respectively.

Third, the resulting 2N samples are multiplied with the synthesis windowto form the windowed IMDCT output.

Thus, a concatenation of the windowed MDCT and the windowed IMDCT may beefficiently carried out in embodiments by the sequence of the first andsecond steps of the windowed MDCT and the second and third steps of thewindowed IMDCT. The third step of the MDCT and the first step of theIMDCT can be omitted entirely in embodiments because they are inverseoperations with respect to each other and thus cancel out. The remainingsteps can be carried out in the time domain only, and thus embodimentsusing this approach can be substantially low in computationalcomplexity.

For one block of MDCT and consecutive IMDCT, the second and third stepof the MDCT and the second and third step of the IMDCT can be written asa multiplication with the following sparse 2N×2N matrix.

$\begin{bmatrix}1 & \; & \; & {- 1} & 0 & \cdots & \cdots & 0 \\\; & \ddots & ⋰ & \; & \vdots & \; & \; & \vdots \\\; & ⋰ & \ddots & \; & \vdots & \; & \; & \vdots \\{- 1} & \; & \; & 1 & 0 & \cdots & \cdots & 0 \\0 & \cdots & \cdots & 0 & 1 & \; & \; & 1 \\\vdots & \; & \; & \vdots & \; & \ddots & ⋰ & \; \\\vdots & \; & \; & \vdots & \; & ⋰ & \ddots & \; \\0 & \cdots & \cdots & 0 & 1 & \; & \; & 1\end{bmatrix}\quad$

In other words, the calculator 129 can be adapted for segmenting theoutput of the time-domain decoder 110 in calculator segments comprising2N sequential samples, applying weights to the 2N samples according toan analysis windowing function, subtracting the first N/2 samples inreversed order from the second N/2 samples, and the last N/2 samples inreversed order to the third N/2 samples, inverting the second and thirdN/2 samples, replacing the first N/2 samples with the time-reversed andinverted version of the second N/2 samples, replacing the fourth N/2samples with the time reversed version of the third N/2 samples, andapplying weights to the 2N samples according to a synthesis windowingfunction.

In other embodiments the overlap/add-combiner 130 can be adapted forapplying weights according to a synthesis windowing function tooverlapping time-domain data blocks provided by the frequency-domain totime-domain converter 128. Furthermore, the overlap/add-combiner 130 canbe adapted for applying weights according to a synthesis windowingfunction being adapted to the size of an overlapping region ofconsecutive overlapping time-domain data blocks.

The calculator 129 may be adapted for applying weights to the 2N samplesaccording to an analysis windowing function being adapted to the size ofan overlapping region of consecutive overlapping time-domain data blocksand the calculator may be further adapted for applying weights to the 2Nsamples according to a synthesis window function being adapted to thesize of the overlapping region.

In embodiments the size of an overlapping region of two consecutivetime-domain data blocks which are encoded in the frequency-domain can belarger than the size of an overlapping of two consecutive time-domaindata blocks of which one being encoded in the frequency domain and onebeing encoded in the time domain.

In embodiments, the size of the data segments can be adapted to the sizeof the overlapping regions. Embodiments may have an efficientimplementation of a combined MDCT/IMDCT processing, i.e. a block TDAcomprising the operations of analysis windowing, folding and unfolding,and synthesis windowing. Moreover, in embodiments some of these stepsmay be partially or fully combined in an actual implementation.

Another embodiment of an apparatus 100 as shown in FIG. 1f illustratesthat an apparatus 100 may further comprise a bypass 140 for theprocessor 120 and the overlay/add-combiner 130 being adapted forbypassing the processor 120 and the overlay/add-combiner 130 whennon-overlapping consecutive time-domain data blocks occur in datasegments, which are encoded in the time domain. If multiple datasegments are encoded in the time domain, i.e. no conversion to thefrequency domain may be useful for decoding consecutive data segments,they may be transmitted without any overlapping. For these cases theembodiments as shown in FIG. 1f may bypass the processor 120 and theoverlap/add-combiner 130. In embodiments the overlapping of blocks canbe determined according to the AAC-specifications.

FIG. 2a shows an embodiment of an apparatus 200 for generating anencoded data stream based on a time-domain data stream, the time-domaindata stream having samples of a signal. The time-domain data streamcould, for example, correspond to an audio signal, comprising speechsections and music sections or both at the same time. The apparatus 200comprises a segment processor 210 for providing data segments from thedata stream, two consecutive data segments having a first or a secondoverlapping region, the second overlapping region being smaller than thefirst overlapping region. The apparatus 200 further comprises atime-domain encoder 220 for encoding a data segment in the time domainand a frequency-domain encoder 230 for applying weights to samples ofthe time-domain data stream according to a first or a second windowingfunction to obtain a windowed data segment, the first and secondwindowing functions being adapted to the first and second overlappingregions and for encoding the windowed data segment in the frequencydomain.

Furthermore, the apparatus 200 comprises a time-domain data analyzer 240for determining a transmission indication associated with a data segmentand a controller 250 for controlling the apparatus such that for datasegments having a first transition indication, output data of thetime-domain encoder 220 is included in the encoded data stream and fordata segments having a second transition indication, output data of thefrequency-domain encoder 230 is included in the encoded data stream.

In embodiments the time-domain data analyzer 240 may be adapted fordetermining the transition indication from the time-domain data streamor from data segments provided by the segment processor 210. Theseembodiments are indicated in FIG. 2b . In FIG. 2b it is illustrated thatthe time-domain data analyzer 240 may be coupled to the input of thesegment processor 210 in order to determine the transition indicationfrom the time-domain data stream. In another embodiment the time-domaindata analyzer 240 may be coupled to the output of the segment processor210 in order to determine the transition indication from the datasegments. In embodiments the time-domain data analyzer 240 can becoupled directly to the segment processor 210 in order to determine thetransition indication from data provided directly by the segmentprocessor. These embodiments are indicated by the dotted lines in FIG. 2b.

In embodiments the time-domain data analyzer 240 can be adapted fordetermining a transition measure, the transition measure being based ona level of transience in the time-domain data stream or the datasegments wherein the transition indicator may indicate whether the levelof transience exceeds a predetermined threshold.

FIG. 2c shows another embodiment of the apparatus 200. In theembodiments shown in FIG. 2c the segment processor 210 can be adaptedfor providing data segments with the first and the second overlappingregions, the time-domain encoder 220 can be adapted for encoding alldata segments, the frequency-domain encoder 230 may be adapted forencoding all windowed data segments and the controller 250 can beadapted for controlling the time-domain encoder 220 and thefrequency-domain encoder 220 and the frequency-domain encoder 230 suchthat for data segments having a first transition indication, output dataof the time-domain encoder 220 is included in the encoded data streamand for data segments having a second transition indication, output dataof the frequency-domain encoder 230 is included in the encoded datastream. In other embodiments both output data of the time-domain encoder220 and the frequency-domain encoder 230 may be included in the encodeddata stream. The transition indicator may be indicating whether a datasegment is rather associated or correlated with a speech signal or witha music signal. In embodiments the frequency-domain encoder 230 may beused for more music-like data segments and the time-domain encoder 220may be used for more speech-like data segments. In embodiments parallelencoding may be utilized, e.g. for a speech-like audio signal havingbackground music.

In the embodiment depicted in FIG. 2c , multiple possibilities areconceivable for the controller 250 to control the multiple componentswithin the apparatus 200. The different possibilities are indicated bydotted lines in FIG. 2c . For example, the controller 250 could becoupled to the time-domain encoder 220 and the frequency-domain encoder230 in order to choose which encoder should produce an encoded outputbased on the transition indication. In another embodiment the controller250 may control a switch at the outputs of the time-domain encoder 220and the frequency-domain encoder 230.

In such an embodiment both the time-domain encoder 220 and thefrequency-domain encoder 230 may encode all data segments and thecontroller 250 may be adapted for choosing via said switch which iscoupled to the outputs of the encoders, which encoded data segmentshould be included in the encoded data stream, based on codingefficiency, respectively the transition indication. In other embodimentsthe controller 250 can be adapted for controlling the segment processor210 for providing the data segments either to the time-domain encoder220 or the frequency-domain encoder 230. The controller 250 may alsocontrol the segment processor 210 in order to set overlapping regionsfor a data segment. In other embodiments the controller 250 may beadapted for controlling a switch between the segment processor 210 andthe time-domain encoder 220, respectively the frequency-domain encoder230. The controller 250 could then influence the switch so to directdata segments to either one of the encoders, respectively to both. Thecontroller 250 can be further adapted to set the windowing functions forthe frequency-domain encoder 230 along with the overlapping regions andcoding strategies.

Moreover, in embodiments the frequency-domain encoder 230 can be adaptedfor applying weights of window functions according to AACspecifications. The frequency-domain encoder 230 can be adapted forconverting a windowed data segment to the frequency domain to obtain afrequency-domain data segment. Moreover, the frequency domain encoder230 can be adapted for quantizing the frequency-domain data segmentsand, furthermore, the frequency-domain encoder 230 may be adapted forevaluating the frequency-domain data segments according to a perceptualmodel.

The frequency-domain encoder 230 can be adapted for utilizing a cosinemodulated filterbank, an extended lapped transform, a low-delayfilterbank or a polyphase filterbank to obtain the frequency-domain datasegments.

The frequency-domain encoder 230 may be adapted for utilizing an MDCT toobtain the frequency data segments. The time-domain encoder 220 can beadapted for using a prediction model for encoding the data segments.

In embodiments where an MDCT in the frequency-domain encoder 230operates in a so-called long block mode, i.e. the regular mode ofoperation that is used for coding non-transient input signals, compareAAC-specifications, the overhead introduced by the switching process maybe high. This can be true for the cases where only one frame, i.e. alength/framing rate of N samples, should be coded using the time-domainencoder 220 instead of the frequency-domain encoder 230.

Then all the input values for the MDCT may have to be encoded with thetime-domain encoder 220, i.e. 2N samples are available at the output ofthe time-domain decoder 110. Thus, an overhead of N additional samplescould be introduced. FIGS. 3a to 3d illustrate some conceivableoverlapping regions of segments, respectively applicable windowingfunctions. 2N samples may have to be coded with the time-domain encoder220 in order to replace one block of frequency-domain encoded data. FIG.3a illustrates an example, where frequency-domain encoded data blocksuse a solid line, and time-domain encoded data uses a dotted line.Underneath the windowing functions data segments are depicted which canbe encoded in the frequency domain (solid boxes) or in the time domain(dotted boxes). This representation will be referred to in FIGS. 3b to3d as well.

FIG. 3a illustrates the case where data is encoded in the frequencydomain, interrupted by one data segment which is encoded in the timedomain, and the data segment after it is encoded in the frequency domainagain. In order to provide the time-domain data which is useful tocancel the time-domain aliasing evoked by the frequency-domain encoder230, when switching from the frequency domain to the time domain, halfof a segment size of overlapping may be used, the same holds fromswitching back from the time domain to the frequency domain. Assumingthat the time-domain encoded data segment in FIG. 3a has a size of 2N,then at its start and at the end it overlaps with the frequency-domainencoded data by N/2 samples.

In case more than one subsequent frames can be encoded using thetime-domain encoder 220, the overhead for the time-domain encodedsection stays at N samples. As it is illustrated in FIG. 3b where twoconsecutive frames are encoded in the time domain and the overlappingregions at the beginning and the end of the time-domain encoded sectionshave the same overlap as it was explained with respect to FIG. 3a . FIG.3b shows the overlap structure in case of two frames encoded withtime-domain encoder 220. 3N samples have to be coded with thetime-domain encoder 220 in this case.

This overhead can be reduced in embodiments by utilizing windowswitching, for example, according to the structure which is used in AAC.FIG. 3c illustrates a typical sequence of Long, Start, 8 Short and Stopwindows, as they are used in AAC. From FIG. 3c it can be seen that thewindow sizes, the data segment sizes and, consequently, the size of theoverlapping regions change with the different windows. The sequencedepicted in FIG. 3c is an example for the sequence mentioned above.

Embodiments should not be limited to windows of the size of AAC windows,however, embodiments take advantage of windows with differentoverlapping regions and also of windows of different durations. Inembodiments transitions to and from short windows may utilize a reducedoverlap as, for example, disclosed in Bernd Edler, “Codierung vonAudiosignalen mit überlappender Transformation and adaptivenFensterfunktionen”, Frequenz, Vol. 43, No. 9, p. 252-256, September 1989and Generic Coding of Moving Pictures and Associated Audio: AdvancedAudio Coding, International Standard 13818-7, ISO/IEC JTC1/SC29/WG11Moving Pictures Expert Group, 1997 may be used in embodiments to reducethe overhead for the transitions to and from the time-domain encodedregions, as it is illustrated in FIG. 3d . FIG. 3d illustrates four datasegments, of which the first two and the last one are encoded in thefrequency domain and the third one is encoded in the time domain. Whenswitching from the frequency domain to the time domain different windowswith the reduced overlapping size are used, therewith reducing theoverhead.

In embodiments the transition may be based on Start and Stop windowsidentical to the ones used in AAC. The corresponding windows for thetransitions to and from the time-domain encoded regions are windows withonly small regions of overlap. As a consequence, the overhead, i.e. thenumber of additional values to be transmitted due to the switchingprocess decreases substantially. Generally, the overhead may beN_(ovl)/2 for each transition with the window overlap of N_(ovl)samples. Thus, a transition with the regular fully-overlapped windowlike an AAC with N_(ovl)=1024 incurs an overhead of 1024/2=512 samplesfor the left, i.e. the fade-in window, and 1024/2=512 samples for theright, i.e. the fade-out window, transition resulting in a totaloverhead of 1024 (=N) samples. Choosing a reduced overlap window likethe AAC Short block windows with N_(ovl)=128 only results in an overalloverhead of 128 samples.

Embodiments may utilize a filterbank in the frequency-domain encoder 230as, for example, the widely used MDCT filterbank, however, otherembodiments may also be used with frequency-domain codecs based on othercosine-modulated filterbanks. This may comprise the derivates of theMDCT, such as extended lapped transforms or low-delay filterbanks aswell as polyphase filterbanks, such as, for example, the one used inMPEG-1-Layer-1/2/3 audio codecs. In embodiments efficient implementationof a forward/back-filterbank operation may take into account a specifictype of window and folding/unfolding used in the filterbank. For everytype of modulated filterbank the analysis stage may be implementedefficiently by a preprocessing step and a block transform, i.e. DCT-likeor DFT, for the modulation. In embodiments the corresponding synthesisstage can be implemented using the corresponding inverse transform and apost processing step. Embodiments may only use the pre- and postprocessing steps for the time-domain encoded signal portions.

Embodiments of the present invention provide the advantage that a bettercode efficiency can be achieved, since switching between a time-domainencoder 220 and the frequency-domain encoder 230 can be done introducingvery low overhead. In signal sections of subsequent time-domain encodingonly, overlap may be omitted completely in embodiments. Embodiments ofthe apparatus 100 enable the according decoding of the encoded datastream.

Embodiments therewith provide the advantage that a lower coding rate canbe achieved for the same quality of, for example, an audio signal,respectively a higher quality can be achieved with the same coding rate,as the respective encoders can be adapted to the transience in the audiosignal.

Depending on certain implementation requirements of the inventivemethods, the inventive methods can be implemented in hardware or insoftware. The implementation can be performed using a digital storagemedium, in particular a disc, DVD or CD having electronically storedcontrol signals stored thereon, which corporate with the programmablecomputer system such that the inventive methods are performed.Generally, the present invention is, therefore, a computer programproduct having a program code stored on a machine-readable carrier, theprogram code being operative for performing the inventive methods whenthe computer program product runs on a computer. In other words, theinventive methods are, therefore, a computer program having a programcode for performing at least one of the inventive methods when thecomputer program runs on a computer.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutationsand equivalents as fall within the true spirit and scope of the presentinvention.

The invention claimed is:
 1. An apparatus for decoding data segments representing a time-domain data stream, wherein one or more data segments are encoded in the time domain, and wherein one or more data segments are encoded in the frequency domain, wherein the data segments which are encoded in the frequency domain represent time-domain data samples of overlapping data blocks, the apparatus comprising: a time-domain decoder adapted to decode a data segment which is encoded in the time domain, to acquire output data of the time-domain decoder; a processor adapted to process the data segments which are encoded in the frequency domain and the output data of the time-domain decoder to obtain time-domain data blocks such that time-domain data blocks obtained based on consecutive data segments which consecutive data segments are encoded in the frequency domain are overlapping time-domain data blocks, and such that consecutive time-domain data blocks of which one is encoded in the frequency domain and of which one is encoded in the time domain, overlap; and an overlap/add-combiner adapted to combine the overlapping time-domain data blocks to obtain the decoded data segments of the time-domain data stream; wherein the overlap/add-combiner is adapted to apply weights according to synthesis windowing functions to overlapping time-domain data blocks, wherein the synthesis windowing function is adapted to a size of an overlapping region of consecutive overlapping time-domain data blocks, wherein a window with a reduced overlapping size is applied to a time-domain data block encoded in the frequency domain when switching from the frequency-domain to the time domain or from the time-domain to the frequency domain; wherein a size of an overlapping region of two consecutive time-domain data blocks which are encoded in the frequency-domain is larger than a size of an overlapping region of two consecutive time-domain data blocks of which one is encoded in the frequency-domain and one is encoded in the time domain.
 2. The apparatus of claim 1, wherein the processor comprises a frequency-domain decoder for decoding data segments which are encoded in the frequency domain to obtain frequency-domain data segments.
 3. The apparatus of claim 1, wherein the processor is adapted for processing a data segment which is encoded in the time domain and in the frequency domain in parallel.
 4. The apparatus of claim 2, wherein the processor comprises a time-domain to frequency-domain converter for converting the output data of the time-domain decoder to obtain converted frequency-domain data segments.
 5. The apparatus of claim 4, wherein the processor comprises a frequency-domain combiner for combining the frequency-domain data segments and the converted frequency-domain data segments to obtain a frequency-domain data stream.
 6. The apparatus of claim 5, wherein the processor comprises a frequency-domain to time-domain converter for converting the frequency-domain data stream to overlapping time-domain data blocks.
 7. The apparatus of claim 2, wherein the frequency domain decoder further comprises a re-quantization stage.
 8. The apparatus of claim 4, wherein the time-domain to frequency-domain converter comprises a cosine modulated filterbank, an extended lapped transform, a low-delay filterbank, a polyphase filterbank or a modified discrete cosine transform.
 9. The apparatus of claim 5, wherein the frequency-domain combiner comprises an adder.
 10. The apparatus of claim 6, wherein the frequency-domain to time-domain converter comprises a cosine modulated filterbank or an inverse modified discrete cosine transform.
 11. The apparatus of claim 1, wherein the time-domain decoder is adapted for using a prediction filter to decode a data segment encoded in the time domain.
 12. The apparatus of claim 1, wherein the processor comprises a calculator for calculating overlapping time-domain data blocks based on the output data of the time-domain decoder.
 13. The apparatus of claim 12, wherein the calculator is adapted for reproducing an overlapping property of the frequency-domain to time-domain converter based on the output data of the time-domain decoder.
 14. The apparatus of claim 13, wherein the calculator is adapted for reproducing a time-domain aliasing characteristic of the frequency-domain to time-domain converter based on the output data of the time-domain decoder.
 15. The apparatus of claim 6, wherein the frequency-domain to time-domain converter is adapted for converting the frequency-domain data segments provided by the frequency-domain decoder to overlapping time-domain data blocks.
 16. The apparatus of claim 15, wherein the overlap/add-combiner is adapted for combining the overlapping time-domain data blocks provided by the frequency-domain to time-domain converter and the calculator to obtain decoded data segments of the time-domain data stream.
 17. The apparatus of claim 8, wherein the calculator comprises a time-domain aliasing stage for time-aliasing output data of the time-domain decoder to obtain the overlapping time-domain data blocks.
 18. The apparatus of claim 12, wherein the calculator is adapted for segmenting the output of the time-domain decoder in calculator segments comprising 2N sequential samples, applying weights to the 2N samples according to an analysis window function, subtracting the first N/2 samples in reversed order from the second N/2 samples, adding the last N/2 samples in reversed order to third N/2 samples, inverting the second and third N/2 samples, replacing the first N/2 samples with the time-reversed and inverted version of the second N/2 samples, replacing the fourth N/2 samples with the time-reversed version of the third N/2 samples, and applying weights to the 2N samples according to a synthesis windowing function.
 19. The apparatus of claim 6, wherein the overlap/add-combiner is adapted for applying weights according to a synthesis windowing function to overlapping time-domain data blocks provided by the frequency-domain to time-domain converter.
 20. The apparatus of claim 19, wherein the apparatus comprises a calculator which is adapted for applying weights to the 2N samples according to an analysis windowing function being adapted to a size of an overlapping region of consecutive overlapping time-domain data blocks and wherein the calculator is adapted for applying weights to the 2N samples according to a synthesis windowing function being adapted to the size of the overlapping region.
 21. The apparatus of claim 1, wherein the overlapping of data blocks is being determined according to the AAC-specifications.
 22. The apparatus of claim 1, further comprising a bypass for the processor and the overlap/add-combiner, the bypass being adapted for bypassing the processor and the overlap/add-combiner when non-overlapping consecutive time-domain data blocks incur in data segments which are encoded in the time domain.
 23. A method for decoding data segments representing a time-domain data stream, wherein one or more data segments are encoded in the time domain, and one or more data segments are encoded in the frequency domain, wherein the data segments which are encoded in the frequency domain represent time-domain data samples of overlapping data blocks, comprising: decoding a data segment which is encoded in the time domain, to obtain output data of a time-domain decoding; processing one of the one or more data segments which is encoded in the frequency domain and the output data of the time-domain decoding to obtain overlapping time-domain data blocks, such that time-domain data blocks obtained based on consecutive data segments which consecutive data segments are encoded in the frequency domain overlap, and such that consecutive time-domain data blocks of which one is encoded in the frequency domain and of which one is encoded in the time domain, overlap; and combining the overlapping time-domain data blocks to obtain decoded data segments of the time-domain data stream; wherein weights according to synthesis window functions are applied to the overlapping time-domain data blocks; wherein the synthesis windowing function is adapted to a size of an overlapping region of consecutive overlapping time-domain data blocks; wherein a window with a reduced overlapping size is applied to a time-domain data block encoded in the frequency domain when switching from the frequency-domain to the time domain or from the time-domain to the frequency domain; wherein a size of an overlapping region of two consecutive time-domain data blocks which are encoded in the frequency-domain is larger than a size of an overlapping region of two consecutive time-domain data blocks of which one is encoded in the frequency-domain and one is encoded in the time domain; wherein the method is performed using a hardware apparatus, or using a computer, or using a combination of a hardware apparatus and a computer.
 24. A non-transitory digital storage medium comprising a computer program for performing the method of claim 23, when the program code runs on a computer.
 25. An apparatus for generating an encoded data stream based on a time-domain data stream, the time-domain data stream having samples of a signal, the apparatus comprising: a segment processor adapted to provide data segments from the data stream; a time-domain encoder adapted to encode at least one windowed data segment in the time domain; a frequency-domain encoder adapted to apply weights to samples of the time-domain data stream according to a first or second windowing function to obtain a windowed data segment, wherein the first and second windowing functions are adapted to overlapping regions of different lengths, wherein the frequency-domain encoder is adapted for encoding a windowed data segment in the frequency domain; a time-domain data analyzer adapted to determine a transition indication associated with a data segment; and a controller adapted to control the apparatus such that for data segments having a first transition indication output data of the time-domain encoder is included in the encoded data stream and for data segments having a second transition indication, output data of the frequency-domain encoder is included in the encoded data stream; wherein the controller is adapted to set the windowing functions for the frequency-domain encoder, such that a window with a reduced overlapping size is used when switching from the frequency-domain to the time-domain or from the time-domain to the frequency domain.
 26. The apparatus according to claim 25, wherein the controller is adapted to set the windowing functions for the frequency-domain encoder such that a size of an overlapping region of two consecutive windowed data segments which are encoded in the frequency-domain, is larger than a size of an overlapping region of two consecutive windowed data segments, of which one is encoded in the frequency-domain and one is encoded in the time-domain.
 27. The apparatus of claim 25, wherein the time-domain data analyzer is adapted for determining the transition indication from the time-domain data stream, the data segments or from data directly provided by the segment processor.
 28. The apparatus of claim 25, wherein the time-domain data analyzer is adapted for determining a transition measure, the transition measure being based on the level of transience in the time-domain data stream or the data segment and wherein the transition indicator indicates whether a level of transience exceeds a predetermined threshold.
 29. The apparatus of claim 25, wherein the segment processor is adapted for providing data segments with overlapping regions of different lengths, the time-domain encoder is adapted for encoding the data segments, the frequency-domain encoder is adapted for encoding windowed data segments, and the controller is adapted for controlling the time-domain encoder and the frequency-domain encoder such that for data segments having a first transition indication output data of the time-domain encoder is included in the encoded data stream and for windowed data segments having a second transition indication output data of the frequency-domain encoder is included in the encoded data stream.
 30. The apparatus of claim 25, wherein the controller is adapted for controlling the segment processor for providing the data segments either to the time-domain encoder or the frequency-domain encoder.
 31. The apparatus of claim 25, wherein the frequency-domain encoder is adapted for applying weights of windowing functions according to the AAC-specifications.
 32. The apparatus of claim 25, wherein the frequency-domain encoder is adapted for converting a windowed data segment to the frequency domain to obtain a frequency-domain data segment.
 33. The apparatus of claim 32, wherein the frequency-domain encoder is adapted for quantizing the frequency-domain data segment.
 34. The apparatus of claim 33, wherein the frequency-domain encoder is adapted for evaluating the frequency-domain data segment according to a perceptual model.
 35. The apparatus of claim 34, wherein the frequency-domain encoder is adapted for utilizing a cosine-modulated filterbank, an extended lapped transform, a low-delay filterbank or a polyphase filterbank to obtain the frequency-domain data segments.
 36. The apparatus of claim 32, wherein the frequency-domain encoder is adapted for utilizing a modified discrete cosine transform to obtain the frequency-domain data segments.
 37. The apparatus of claim 25, wherein the time-domain encoder is adapted for using a prediction filter for encoding the data segments.
 38. A method for generating an encoded data stream based on a time-domain data stream, the time-domain data stream having samples of a signal, comprising: providing data segments from the data stream; determining a transition indication associated with the data segments; encoding a data segment in the time domain; applying weights to samples of the time-domain data stream according to a first or a second windowing function to obtain a windowed data segment, wherein the first and second windowing functions are adapted to overlapping regions of different lengths and encoding the windowed data segment in the frequency domain; and controlling such that for data segments having a first transition indication output data which are encoded in the time-domain is included in the encoded data stream and for data segments having a second transition indication output data which are encoded in the frequency domain is included in the encoded data stream; wherein the windowing functions are set such that a window with a reduced overlapping size is used when switching from the frequency-domain to the time-domain or from the time-domain to the frequency domain.
 39. A non-transitory digital storage medium comprising a computer program having a program code for performing the method of claim 38, when the program code runs on a computer. 